As is known, electro-optical modulators are of importance in optical telecommunications systems. They, on the basis of an electrical command, absorb or transmit optical pulses sent from a suitable source, namely a laser, towards their surface. At their output duly coded digital signals can be obtained to be transmitted by optical fibers.
Present-day optical modulators exploit different physical effect, e.g. magneto-optical, acousto-optical and electro-optical effects. The latter effect, more particularly, is used in the so-called "quantum-well" modulators. These devices consist of a succession of layers of semiconductor material, a thin layer of which (having a thickness lower than 250 .ANG.) forms the quantum well. The two layers by which it is sandwiched, made of chemically-different semiconductor materials, form the barrier layers.
This arrangement can not as an electro-optical modulator whenever it is incorporated in an electric field perpendicular to the thin layer. To this end the three layers are comprised between semiconductor layers which in turn contact metal layers, to which a suitable potential difference can be applied. The metal layers allow the creation of an electric field, but do not influence the physical phenomenon of absorption or transmission of light radiation, dominated by the thin layer and by the confining barrier layers.
The phenomenon, known as the Stark effect, is an energy level shift induced by an electrical field in the thin layer material. In fact, inside the quantum well there is a set of discrete and quantized levels in the valence band as well as a set of levels in the conduction band. Electron transitions from valence band to conduction band take place by foton absorption with an energy corresponding to an energy level difference. When an electric field is applied, both in the valence band and in the conduction band an energy level shift occurs and consequently, a shift in the light absorption threshold of the device. More particularly, in the known devices the absorption threshold shifts towards lower energy levels, corresponding to longer wavelengths (red shift). Larger shifts can be obtained by using highly-coupled multiple quantum-well structures. Structures consisting of two quantum wells separated by a very thin barrier layer, about 10A thick, are well known (see "Electroabsorption in GaAs/A1GaAs coupled quantum well waveguides", N.M. Islam et al, Applied Physic Lett. 50 (16), 20l Apr. 1987). Absorption threshold shifts towards lower energies are thus obtained which are about ten times greater than those obtainable with a single quantum well. Also the applied electrical field can be lower: about 8.10.sup.4 V/cm instead of 1.10.sup.5, which is very near the breakdown voltage of the structure. However, electro-optical modulators whose absorption threshold can be shifted towards higher energy levels, corresponding to shorter wavelengths (blue shift) have not previously been described.